Coniferous woods, especially pine, contain among other components, lignin, resin acids and long-chain fatty acids. Crude tall oil, a mixture of the resin and fatty acids, is produced as a chemical by-product of the kraft pulping process.
In the kraft pulping process, wood chips are fed into a digester and a "white" cooking liquor containing sodium hydroxide (NaOH) and sodium sulfide (Na.sub.2 S) is added. The contents are then heated according to a predetermined schedule to complete the cooking reactions, wherein the resin acids and fatty acids are released from the wood chips and saponified. The resulting cooked pulp is separated from the residual cooking liquor, known as black liquor, in a brown stock washing process. The sodium salts of the resin acids and fatty acids, commonly referred to as tall oil soap or "black liquor soap", are suspended in the black liquor. The wood pulp is treated further in the papermaking process to produce papers of various grades.
The recovery of chemicals from the black liquor, the reconstitution of those chemicals to form fresh cooking liquors, the realization of energy from incineration of organic residuals, and minimization of air and water pollution, are all vital parts of the kraft process. Among those chemicals which are recovered from the black liquor is tall oil soap. The soap is contained in the "weak" black liquor (i.e. unconcentrated) which is recovered in the pulping process as a filtrate from the pulp separation.
The initial weak black liquor (at about 15% solids) is filtered to remove fiber and is passed into a weak liquor skimmer. Thirty per cent (30%) to 70% of the available black liquor soap is skimmed off. The black liquor then passes through several stages of evaporative concentration to raise the solids content to 25-30%. The black liquor then passes through a second skimmer from which the remaining 30-70% of soap is skimmed off. The black liquor finally passes through more stages of evaporative concentration to raise the solids to 50-60%. It is then fed to a recovery furnace to be burned. Burning the organic content of the black liquor provides energy for the paper making process and reconstitutes the inorganic chemicals for reuse in fresh pulping liquors. Foran, C. D., "Black Liquor Soap Recovery Methods Employed by Union Camp," Naval Stores Review, 94, 14 (1984) (Incorporated by reference herein in its entirety).
One skilled in the art of making kraft paper will recognize that there are various techniques for recovering tall oil soap from the kraft black liquor. Frequently, black liquor soap is degassed (or concentrated) to remove foam and is then settled to decant out excess residual black liquor.
The separated tall oil soap is then normally acidulated with sulfuric acid to form crude tall oil, which can be refined by vacuum fractionation to form tall oil fatty acids and tall oil rosin. The fatty acids and rosin are used in numerous industrial applications, including the preparation of polyamides, rosin resins, adhesive and epoxy compositions. Heretofore the acidulation process comprised the addition of sulfuric acid (H.sub.2 SO.sub.4) to the tall oil soap to produce crude tall oil and Na.sub.2 SO.sub.4 salt cake solution. One skilled in the art will recognize that there are various acidulation processes utilizing H.sub.2 SO.sub.4 for the conversion of tall oil soap to crude tall oil. For a more thorough discussion with references, see McSweeney, E. E.; "Sulfate Naval Stores", Naval Stores 171, (1989). The Na.sub.2 SO.sub.4 as described above is recycled to the kraft pulping process to make up for sodium and sulfur losses.
The economics of the production of crude tall oil from tall oil soap depends to a considerable degree on the ability to dispose of the spent acid from the acidulation process. Heretofore, the producers of crude tall oil would sell the by-product Na.sub.2 SO.sub.4, hereinafter referred to as salt cake, to the paper mills as make-up chemicals in the kraft pulping process. A typical kraft mill in the early '70s was designed for cost effective chemical recovery with little regard for the environment. Sodium recovery was around 93% while sulfur recovery was only about 61%. Twiss, A. H., Naval Stores Review, 94 (2), 14 (1984) (Incorporated by reference herein in its entirety.) With the introduction of more stringent environmental controls on the emission of air and water pollutants, a state of the art greenfield mill (1984) being designed to meet the more stringent new source performance standards was predicted to achieve over 97% sodium recovery and 91% sulfur recovery. Twiss reports that as of 1984, total sodium loss expressed as equivalent salt cake had been decreased from 135 to 70 pounds/ton of pulp for a mill meeting then existing source standards, down to 53 pounds for a new state of the art (1984) mill. Total sulfur losses were decreased from 175, to 75, to 40 pounds of equivalent salt cake per ton of pulp, due mainly to reduced emissions from the recovery boiler and lime kiln. These reduced losses have had a drastic impact on the make-up Sodium and sulfur requirements in the kraft pulping process, which must balance the losses. The result is that the paper mills no longer desire or require the previous levels of salt cake produced in the sulfuric acid acidulation of tall oil soap to make-up for losses of sodium and sulfur. In fact, a large amount of the salt cake make-up has been replaced with more costly caustic soda to lower the sulfur make-up. Without the paper mills to use the salt cake produced by the acidulation of tall oil soap, it would appear that the salt cake must be sewered. Organic contaminants contained in the sewer discharge which contains the salt cake increases the biochemical oxygen demand (BOD) levels in the water streams receiving the discharge. Therefore, the limitations on sewering of the salt cake will increase due to even more stringent environmental restrictions.
Wong, also discussed tall oil soap acidulation and sulfur balance problems in kraft mills. Wong, A., Naval Stores Review, 94, (3), 8 (1984) (Incorporated by reference herein in its entirety.) Wong reported the same problems associated with the salt cake from the acidulation of the tall oil soap as did Twiss. Wong proposed the replacement of H.sub.2 SO.sub.4 used in soap acidulation with chlorine dioxide generator effluent to lower the sulfur input into the mill liquor system. However, the reduced sulfur input would still be higher than the losses in the mill by about 10%.
It has been suggested that the tall oil acidulation process can be modified to reduce or eliminate by-product sulfur by the use of other common acids, such as hydrochloric. The use of such acids, however, would prohibit recovery of the spent acid due to build-up of extraneous anions and problems in tall oil purification. Electrolysis of spent acid to NaOH and a mixture of H.sub.2 SO.sub.4 and Na.sub.2 SO.sub.4 has also been investigated. Replacement of about one-half of the H.sub.2 SO.sub.4 with carbon dioxide has also been patented by Bills, A. M., U.S. Pat. No. 3,901,869 (1975) (Twiss, at p. 16).
These proposed solutions only partially address the problem. Very simply, too much sulfur as Na.sub.2 SO.sub.4 is produced in the soap acidulation plants to be recycled to the paper mill liquor system for satisfactory sulfidity and sulfur emissions control.
Another problem associated with the reduced requirement for Na.sub.2 SO.sub.4 salt cake as a make-up chemical in the kraft pulping process is that caustic soda must be used to replace sodium lost in the kraft pulping process. Not only is the caustic soda more expensive than the H.sub.2 SO.sub.4 acidulation salt cake by-product, but the production of NaOH from sodium chloride yields chlorine. If the pulp manufacturer elects to buy only the NaOH and not the chlorine, a premium price is paid for the NaOH since the producer of the NaOH and chlorine must then sell or dispose of the chlorine himself. In essence, the papermaker is paying both for the caustic soda and the chlorine, even though he does not use the chlorine.
There is a long felt need in the papermaking industry for a process to acidulate tall oil soap to form crude tall oil without the formation of Na.sub.2 SO.sub.4 salt cake. There is also a need to economically produce caustic soda to be recycled to the pulping process without the production of chlorine. These needs have not been satisfied to date, even in the face of more stringent environmental emission controls and reduced requirements for salt cake in the paper mills.
Therefore it is an object of the present invention to provide a process for the acidulation of tall oil soap to form crude tall oil without the formation of Na.sub.2 SO.sub.4 salt cake. It is another object of the present invention to provide a process for the economical production of caustic soda to be recycled to the kraft pulping process without the production of chlorine.